Fluidic directional control valve assembly

ABSTRACT

Directional control valve having a ported body with a chamber, a spool reciprocable in the chamber, a flow passageway extending between a supply port and each end of the spool chamber, selectively operable valves in each flow passageway for alternately venting and pressurizing its respective end of the spool chamber, and a pair of spool balancing members engageable respectively with opposite ends of the spool and continuously urged under a biasing force of fluid under supply line pressure toward engagement with their respective ends of the spool.

Padula [75] Inventor: Lawrence Dominic Padula, New Britain, Conn.

[73] Assignee: Skinner Precision industries, Inc.,

New Britain, Conn.

[22] Filed: Nov. 6, 1972 [211 Appl. No.: 304,213

[52] US. Cl. 137/625.64 [51] Int. Cl. Fl6r 11/07 [58] Field of Searchl37/625.6625.69

[56] References Cited UNITED STATES PATENTS 3,315,702 4/1967 Passagiol37/625.64

FLUIDIC DIRECTIONAL CONTROL VALVE ASSEMBLY [451 Nov. 13, 1973 3,323,4216/1967 Olmstead l37/625.64 X

Primary ExaminerHenry T. Klinksiek Attorney-John M. Prutzman et al.

[57] ABSTRACT Directional control valve having a ported body with achamber, a spool reciprocable in the chamber, a flow passagewayextending between a supply port and each end of the spool chamber,selectively operable valves in each flow passageway for alternatelyventing and pressurizing its respective end of the spool chamber, and apair of spool balancing members engageable respectively with oppositeends of the spool and continuously urged under a biasing force of fluidunder supply line pressure toward engagement with their respective endsof the spool.

11 Claims, 3 Drawing Figures FLUIDIC DIRECTIONAL CONTROL VALVE ASSEMBLYFIELD OF THE INVENTION This invention generally relates to fluid valvesand particularly concerns fluidic directional control valves of ageneral type described in my US. Pat. No. 3,523,555 entitled SolenoidControlled Four-Way Valve issued Aug. 11, 1970 and assigned to theassignee of this invention.

BACKGROUND OF THE INVENTION As requirements for high speed spool valveoperation increases with increasing axial forces at maximum fluid flowrates, seemingly incompatible objects of reduced minimum operatingpressures are encountered, sometimes with further requirements for lowwattage operation, e.g., of a solenoid valve operator. Spring return ofa valve spool to normal rest position frequently requires that thespring rate is high enough to effectively overcome frictional forcesimposed on pressure surface areas of the valve spool for effecting spoolmovement as well as the maximum ring frictional forces for sealing thespool. These 0" ring frictional forces are highest when the inletpressure is at its maximum and, accordingly, require significantlygreater forces to center the spool at maximum rated inlet pressure. Thisconventionally results in typical valve assemblies having increasinglylarger springs for applying correspondingly larger spool return forces.Increasing the size of the return spring normally results in a valveassembly design of an overall increased size because of the requiredincreased spring chamber space to accommodate the larger spring and,normally, a larger diameter actuating piston on the valve spool for agiven minimum operating pressure. As a result, a larger solenoid valveoperator is also required to operate the larger diameter actuatingpiston with equal speed, whereby the overall package size is undesirablyincreased with concomitant increases in costs of manufacture.

OBJECTS OF THE INVENTION Accordingly, a primary object of this inventionis to provide a new and improved fluidic directional control valveassembly capable of high speed valve operation under flow capacityconditions with minimum operating power requirements while at the sametime achieving the seemingly incompatible aims of reduced minimumoperating pressure in a compact assembly.

Another object of this invention is to provide such a new and improvedvalve assembly which is capable of providing long run reliability in adesign quick and easy to manufacture and assemble at economical costswith minimum service requirements over an extended operating life.

Other objects will be in part obvious and in part pointed out in moredetail hereinafter.

SUMMARY OF THE INVENTION The valve assembly of this invention achievesthe foregoing objects by the provision of auxiliary fluid springcompartments in the valve body at opposite ends of its spool chamberwith the compartments in constant communication with the supply port foreffecting a centering fluid bias on a pair of balancing pistonsengageable with opposite ends of the spool for continuously urging itinto a center rest position. The auxiliary fluid spring compartmentsadditionally contain a spring which assists the fluid pressure forces onits respective spool balancing piston, whereby the centering force ofsuch springs is sufficiently high to overcome the actuating pistonfrictional forces and the minimum 0 ring sealing frictional forces onthe spool. with zero gauge pressure at the supply port. The additionalforce requirements for centering the spool under applied inlet pressureoperation is provided by the centering piston force effected by thefluid pressure forces acting on the centering piston which isproportional to the inlet supply line pressure.

A better understanding of the objects, advantages, features, propertiesand relationships of the invention will be obtained from the followingdetailed description and accompanying drawings which set forth anillustrative embodiment and is indicative of the ways in which theprinciple of this invention is employed.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a side elevational view, partly broken away and partly insection, showing a preferred embodiment of a valve assembly constructedin accordance with this invention;

FIG. 2 is a partial longitudinal schematic view on a reduced scaleshowing fluid flow with the valve assembly in a first energizedcondition; and

FIG. 3 is a view similar to FIG. 2 with the valve as sembly in a secondenergized condition.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE. INVENTIONReferring to the drawing wherein a preferred embodiment of a valveassembly 10 of this invention is illustrated, a generally rectangularvalve body 12 is shown having a cylindrical valve chamber 14 wherein avalve spool 16, preferably formed of high strength I aluminum with apolished hardcoat surface, is received for axial reciprocating movementsresponsive to alternate operation of two solenoid pilot valve operators18 and 20 of a type particularly suitable for use in valve assembly 10.The solenoid pilot valve operators 18, 20 described in connection withthe preferred embodiment of this invention have been found to worksatisfactorily. However, it is contemplated that other piloting may beutilized. Suitable modifications may be provided, for example, inutilizing external pilot pressure in separate auxiliary lines connectedto thepilot valve operators and, if desired, modification may beprovided for manual actuation of the operators. The solenoid pilot valveoperators 18 and 20 are of identical construction. For an understandingof this invention only one of the operators need be described.

Suitable machine screws, not shown, may be used to secure a base portion21 of operator 18 to the right hand end of body 12 as viewed in thedrawing. Sleeve 22 is secured to base portion 21 by threaded collar'24,

A reciprocable bi-positional armature 40 is received for slidingmovement within sleeve 22, and longitudinally extending grooves such asat 42 along the peripheral surface of armature 40 permit free flow offluid between armature 40 and sleeve 22. Armature 40 has a radial endflange 44. A compression spring 46 seated between flange 44 and aradially outwardly flaired end of sleeve 22 biases armature 40downwardly (as viewed in the drawing) into compartment 32 and against anannular valve seat 48. Valve seat 48 is apertured to connect with aninternal pilot flow passageway 50 leading to a supply port 52 in valvebody 12. In the specifically illustrated embodiment, supply port 52 isshown formed intermediate the ends of body 12 in a central location withpassageway 50 and another internal pilot flow passageway 50A leading inopposite longitudinal directions of body 12 from supply port 52 toopposite ends of body 12 to connect with compartments 32 and 32A of therespective solenoid pilot valve operators 18 and 20.

To selectively apply valve spool operating pressure to opposite ends ofthe valve spool chamber 14 as determined by selective energization ofoperators 18 and 20, opposite ends of the body 12 and connecting baseportions 21 and 21A are formed to provide relatively enlarged endchamber portions or motor cylinders 54 and 56 each having a common inletand exhaust orifice at 58 and 60 which are in direct communication withcompartments 32 and 32A formed in base portions 21 and 21A of theirrespective operators 18 and 20. Cylinders 54, 56 each have a suitablelining 62, 64, preferably formed of stainless steel, and have sealretainers 66, 68 fixed at inner ends of each cylinder 54, 56. Retainers66 and 68 are apertured to respectively surround lands 70 and 72 ofspool 16 and retain seal cages such as at 74 in assembly. In thespecifically illustrated embodiment, cages 74 axially fitted withinchamber 14 about spool 16 with each cage 74 radially aligned with one ofthe ports for retaining the O ring seals such as at 76 in separatedcoaxially positioned relation within chamber 14 for independent sealingengagement with spool 16. Four ports 78, 80, 82 and 84 are shown, inaddition to supply port 52, communicating with chamber 14 with the portsalternately disposed along the length of chamber 14, i.e., with thecentral supply port 52 formed on one side of chamber 14 between twoexhaust ports 78 and 82 respectively servicing two working ports 80 and84 formed on the opposite side of chamber 14.

While spools of different construction and bodies having differentporting arrangements obviously may be utilized to effect properintercommunication between ports, spool 16 in the preferred embodimenthas a third land 86 interposed between the previously mentioned lands 70and 72 and separated by a pair of axially spaced grooves 88 and 90 whichare suitably dimensioned and spaced apart for alternately connectingsupply port 52 to one of the working ports 80, 84 and exhausting theother through its respective exhaust port 78, 82 in accordance with theaxial positioning of spool 16 upon selectively venting and pressurizingthe outer compartments 54A, 56A of cylinders 54, 56 responsive toenergization and selective de-energization of solenoid pilot valveoperators 18, 20.

The seal retainers 66 and 68 are each suitably apertured to connect withbleed openings 92 and 94 in the body 12 for exhausting trapped air aheadof each actuating piston 96 and 98 fixed respectively on the oppositeends of spool 16. Fluid leakage between inner and outer cylindercompartments 54B, 54A and 56B, 56A is minimized, while accommodatingmanufacturing variations without close tolerance requirements, byproviding annular seals 100, 102 fitted about a neck of each actuatingpiston 96, 98 to provide a self-centering piston and spool arrangement.The seals 100, 102 are each formed of a tough resilient material ofC-shaped half-section opening toward the outer cylinder compartments54A, 56A such that upon directing fluid into the outer cylindercompartment to drive the piston axially, the perimeter of each seal 100,102 expands radially outwardly against its liner 62, 64, and the trappedair in the inner cylinder compartment vents to atmosphere. Upon returnof spool 16 to its center blocked port position of FIG. 1, both pilotoperators are deenergized and the outer cylinder compartments 54A, 56Aare exhausted (or vented to atmosphere when air is used as the supplyfluid) through their common inlet and exhaust orifices 58, 60 and alongthe grooves 42 of their respective armature 40 which connect to theexhaust passage 28. During such return movement, the force exerted bythe seals 100, 102 upon the liners 62, 64 approaches zero uponcontraction of the seal material due to their inherent resiliency,thereby reducing the required return force which must be exerted on thespool 16. By virtue of the stainlesssteel construction of the liners 62,64, a low friction surface is presented to the seals 100, 102, whichfurther minimizes frictional resistance to spool movement, and the highstrength aluminum spool construction provides for relatively low massand low inertia to further reduce the minimum operating pressurerequired.

As noted, the specifically illustrated embodiment of this inventionprovides a valve spool and porting arrangement wherein all ports areblocked in the center spool position. To maintain proper centering ofthe spool in accordance with this invention, auxiliary fluid springcompartments 104, 106 are shown provided in the base portions 21, 21A ofeach pilot operator 18, 20 in coaxially aligned relation to the spoolchamber 14 with a balancing member or spool centering piston 108, 110projecting through end openings 112, 114 in the closed ends of thecylinders 54, 56 for engagement with opposite ends of spool 16. Eachcentering piston 108, 110 is fitted for reciprocating movement withinits compartment 104, 106 which is in part provided with a suitable liner116, 118. Each piston 108, 110 has an extension or rod 120, 122projecting through the end opening 112, 114 and is normally biased intoa rest position (FIG. 1) with a radial collar 124, 126 of each piston108, 110 engaging the outer end wall of its respective compartment 54,56 under the influence of a spool centering spring 128, 130. The lattereach have one end seated on an outer wall of its spring compartment 104,106, defined by a bushing 132, 134, and the opposite spring end beingseated on a suitable cap 136, 138 attached to a rearwardly extendingstem of its piston 108, 110. Undesired fluid leakage between the springcompartment 104, 106 and its respective cylinder 54, 56 is effectivelyminimized by the provision of a U cup packing seal 140, 142 fitted aboutthe stem of its piston 108, l 10 and which is of C-shaped half-sectionopening outwardly toward the outer portion of its spring compartment104, 106. Each spool centering spring 128, is selected with a springrate sufficiently high to allow the centering spring force to overcomeany frictional forces imposed on the spool 16 such as the forces exertedby the seals 100, 102 on the actuating pistons 96, 98 in addition to theminimum 0 ring frictional forces under zero gauge pressure conditions,i.e., zero gauge pressure being applied to supply port 52.

In accordance with still another aspect of this invention, theadditional force requirements to ensure long run reliability under flowcapacity conditions in a compact assembly with reduced minimum operatingpressures is achieved by the provision of pilot passage 144, 146directly connecting pilot valve compartments 32, 32A of the flowpassageways 50, 50A and the spring compartment 104, 106. The connectionof pilot passages 144, 146 with the spring compartments 104, 106 isformed at a juncture adjacent the liners 116, 118 within the springcompartments 104, 106 which is beyond the stroke limit position of thecentering pistons 108, 110 upon their being displaced in a directionagainst their respective springs 128, 130 under the influence of thespool movement. The stroke limit position of each centering piston 110,108 is determined by the extreme limit position of the spool 16 from itscenter rest position (FIG. 1) as established by engagement of pistons 98and 96 respectively with the left and right hand end walls of cylinders56 and 54 as viewed in FIGS. 2 and 3.

By virtue of the above described construction, as the supply inletpressure increases, an additional spool actuating force is provided evenunder maximum fluid flow conditions, e.g., by the centering piston forcewhich is directly proportional to the supply line pressure. Accordingly,with the coil of the pilot valve operator 20 de-energized, the commoninlet and exhaust orifice 60 to the outer cylinder compartment 56A isclosed off to the supply port 52 and will be understood to be connectedto exhaust via the exhaust passage of the valve operator 20. The spool16 may then be thrust from its center position to its extreme left handlimit position (FIG. 2) under differential pressure upon energization ofpilot operator 18 whereby the relatively large pressure surface foactuating piston 96 overcomes the force of the spool centering spring130 and the centering force of balancing piston 110 which remains biasedagainst actuating piston 98 under the'influence of the inlet pressure ofthe supply fluid. As piston 96 is thrust to the left, the air ahead ofpiston 96 vents to atmosphere from the inner cylinder compartment 548 toshift spool 16 into its extreme left hand limit position, therebyintroducing supply fluid into working cylinder port 84 and exhaustingworking port 80 through its exhaust port 78 (FIG. 2). It will be seenthat the operating fluid pressure in the spring chambers 104, 106 is ata maximum substantially corresponding to the supply line inlet pressure,thereby additionally effecting a fluid cushion for spool 16 to at leastpartially absorb its impact upon its being shifted into its left handposition against the end wall of its outer cylinder compartment 56Aprovided by the base portion 21A of pilot operator 20. Upon deenergizingpilot valve operator 18, armature 40 returns under spring force into itsillustrated normal position closing off the flow passageway to the outercylinder compartment 54A and exhausting it (or venting it to atmospherewhen air is the supply fluid) through exhaust passage 28 to effect spoolreturn under the influence of spring 130 with the assistance of thereturn fluid bias. With substantially identical balanced spring andfluid biasing forces being exerted on its opposite axial ends, spool 16then is once again in accurately centered relationship to the portedbody 12 to again effect closing and blocking of all ports.

Actuation of the left hand solenoid pilot valve operator 20 is the sameas that described above in connection with the right hand solenoid pilotvalve operator 18 but in reverse relation whereby the spool 16 isshifted into its right hand extreme limit position (FIG. 3) to introducesupply fluid into the other working port and exhaust the previouslypressurized working port 84 through its exhaust port 82.

The valve assembly of this invention effects a balanced centering of thevalve spool under substantially identical but oppositely directed springand fluid biasing forces by virtue of the disclosed auxiliary springchamber arrangement provided at the opposite axial ends of the valvebody. In addition, reliable center positioning of the spool is effectedat any pressure within design limits for long run reliability and formeeting the flow capacity requirements and reduced minimum operatingpressure requirements in a compact assembly capable of high speed valveoperation with minimum power even when high axial forces on the spoolare to be contended with under maximum fluid flow conditions.

' As will be apparent to persons skilled in the art, variousmodifications, adaptations and variations of the foregoing specificdisclosure can be made without departing from the teachings of thepresent invention.

I claim:

l. A valve assembly for connection to a supply source of fluid underpressure and comprising a body having a cylindrical chamber with portsopening into the chamber, a valve spool reciprocable in the chamber forcontrolling fluid flow between ports, fluid passage means for connectingthe supply source with opposite ends of the chamber, selectivelyoperable means for controlling the position of the spool including valvemeans in the passage means for connecting each end of the chamberalternately to exhaust and to the supply source, a compartment formed inthe body adjacent each end of the chamber with each compartment being incontinuous communication with the supply source, and a spool centeringpiston received in each compartment for reciprocation axially of thepath of movement of the spool, the pistons each being biased toward thespool by fluid from the supply source and having an extension projectinginto the chamber for engaging and centering the spool when the ends ofthe chamber are connected to exhaust.

2. The valve assembly of claim 1 further including a spring enclosedwithin each compartment and urging the spool centering piston axiallytoward the spool for engaging and centering the spool.

3. The valve assembly of claim 2 wherein the valve body establishesfirst and second stroke limit positions for the spool on opposite sidesof a center rest position, and wherein the spring in each compartmenthas a spring rate sufficient to overcome any frictional forces exertedon the spool in the absence of differential fluid pressure forces beingapplied to the spool for moving the spool from its first and secondstroke limit positions into its center rest position.

4. The valve assembly of claim 1 wherein the ports include a supply portopening into the chamber, wherein the passage means includes an internalflow passageway formed in the body for connecting the supply port withopposite ends of the chamber, and wherein a pilot passage is providedfor connecting each compartment to the flow passageway at its respectiveend of the body in permanently open communication to the supply port.

5. The valve assembly of claim 4 wherein the pilot passage connectingeach compartment to the supply port is internally formed in the valvebody.

6. The valve assembly of claim 4 wherein the flow passageway includes acommon inlet and exhaust orifice in each end of the chamber, wherein anexhaust passage is provided for communicating each common inlet andexhaust orifice at opposite ends of the chamber to exhaust, and whereinthe valve means includes an independently operable pilot valve member inthe flow passageway at each end of the body for controlling fluid flowthrough the flow passageway to the opposite ends of the chamber.

7. The valve assembly of claim 6 wherein an operator is provided at eachend of the valve body for moving its respective pilot valve memberbetween first and second flow control positions, each pilot valve memberin said first position being in a normally closed position wherein thecommon inlet and exhaust orifice at its respective end of the chamber isconnected to its exhaust passage and is closed to the supply port, eachpilot valve member in said second position being in an open positionwherein the common inlet and exhaust orifice at its respective end ofthe chamber is connected to the supply port and closed to its exhaustpassage.

8. The valve assembly of claim 1 wherein each end of the spool has apiston of enlarged size relative to its spool centering piston with anannular seal of generally C shaped half-section fitted about each pistonon the spool and opening toward its respective end of the chamber toprovide sealing between the ends of the spool and the body, wherein asealing subassembly having a series of O ring seals is mounted withinthe chamber between ports to surround the spool and provide perimetersealing between the spool and the body, and wherein the compartment foreach spool centering piston further includes a spring enclosed withinthe compartment urging the spool centering piston axially toward thespool for engaging the spool, the spring having a spring rate sufficientto overcome any frictional forces exerted by the O ring seals and theannular piston seals on the spool with zero gauge pressure being appliedto the ports.

9. The valve assembly of claim 1 wherein the passage means includes acommon inlet and exhaust orifice in each end of the chamber, and whereinthe valve means comprises a solenoid pilot valve operator on each end ofthe body with each valve operator having an independently andselectively operable pilot valve member mounted in the passage means formovement between an open position, wherein the orifice at its respectiveend of the chamber is in communication with the supply source and shutoff to exhaust, and a closed position, wherein the orifice at itsrespective end of the chamber is connected to exhaust and closed to thesupply source.

10. The valve assembly of claim 9 wherein the pilot valve member at eachend of the body is normally in closed position with the common inlet andexhaust orifices each being connected to exhaust upon deenergization ofboth solenoid pilot valve operators such that the spool may be centeredby the spool centering pistons under the return fluid biases appliedthereto.

11. A valve assembly comprising a body having a cylindrical chamber withports including a supply port opening into the chamber, a valve spoolaxially reciprocable in the chamber between first and second strokelimit positions for controlling fluid flow between ports, the valvespool having a center rest position intermediate said first and secondstroke limit positions, fluid passage means in he body for connectingopposite ends of the chamber to exhaust and to the supply port, valvemeans in the fluid passage means for selectively and independentlyconnecting the ends of the chamber to exhaust and to the supply port, apair of auxiliary compartments in operatively aligned adjacent relationto opposite ends of the chamber, and a pair of spool balancing memberseach having a piston of reduced size relative to the axial ends of thespool with the pistons reciprocably mounted within the compartments,respectively, the spool balancing members each having an extensionprojecting into the adjacent end of the chamber for engaging the spool,the fluid passage means establishing continuous communication betweenthe supply port and the auxiliary compartments for continuously urgingthe balancing members toward engagement with opposite ends of the spooland applying balanced return forces thereon biasing the spool into itscenter rest position.

1. A valve assembly for connection to a supply source of fluid underpressure and comprising a body having a cylindrical chamber with portsopening into the chamber, a valve spool reciprocable in the chamber forcontrolling fluid flow between ports, fluid passage means for connectingthe supply souRce with opposite ends of the chamber, selectivelyoperable means for controlling the position of the spool including valvemeans in the passage means for connecting each end of the chamberalternately to exhaust and to the supply source, a compartment formed inthe body adjacent each end of the chamber with each compartment being incontinuous communication with the supply source, and a spool centeringpiston received in each compartment for reciprocation axially of thepath of movement of the spool, the pistons each being biased toward thespool by fluid from the supply source and having an extension projectinginto the chamber for engaging and centering the spool when the ends ofthe chamber are connected to exhaust.
 2. The valve assembly of claim 1further including a spring enclosed within each compartment and urgingthe spool centering piston axially toward the spool for engaging andcentering the spool.
 3. The valve assembly of claim 2 wherein the valvebody establishes first and second stroke limit positions for the spoolon opposite sides of a center rest position, and wherein the spring ineach compartment has a spring rate sufficient to overcome any frictionalforces exerted on the spool in the absence of differential fluidpressure forces being applied to the spool for moving the spool from itsfirst and second stroke limit positions into its center rest position.4. The valve assembly of claim 1 wherein the ports include a supply portopening into the chamber, wherein the passage means includes an internalflow passageway formed in the body for connecting the supply port withopposite ends of the chamber, and wherein a pilot passage is providedfor connecting each compartment to the flow passageway at its respectiveend of the body in permanently open communication to the supply port. 5.The valve assembly of claim 4 wherein the pilot passage connecting eachcompartment to the supply port is internally formed in the valve body.6. The valve assembly of claim 4 wherein the flow passageway includes acommon inlet and exhaust orifice in each end of the chamber, wherein anexhaust passage is provided for communicating each common inlet andexhaust orifice at opposite ends of the chamber to exhaust, and whereinthe valve means includes an independently operable pilot valve member inthe flow passageway at each end of the body for controlling fluid flowthrough the flow passageway to the opposite ends of the chamber.
 7. Thevalve assembly of claim 6 wherein an operator is provided at each end ofthe valve body for moving its respective pilot valve member betweenfirst and second flow control positions, each pilot valve member in saidfirst position being in a normally closed position wherein the commoninlet and exhaust orifice at its respective end of the chamber isconnected to its exhaust passage and is closed to the supply port, eachpilot valve member in said second position being in an open positionwherein the common inlet and exhaust orifice at its respective end ofthe chamber is connected to the supply port and closed to its exhaustpassage.
 8. The valve assembly of claim 1 wherein each end of the spoolhas a piston of enlarged size relative to its spool centering pistonwith an annular seal of generally ''''C'''' shaped half-section fittedabout each piston on the spool and opening toward its respective end ofthe chamber to provide sealing between the ends of the spool and thebody, wherein a sealing subassembly having a series of ''''O'''' ringseals is mounted within the chamber between ports to surround the spooland provide perimeter sealing between the spool and the body, andwherein the compartment for each spool centering piston further includesa spring enclosed within the compartment urging the spool centeringpiston axially toward the spool for engaging the spool, the springhaving a spring rate sufficient to overcome any frictional forcesexerted by the ''''O'''' ring seals and the annular piston seals on thespoOl with zero gauge pressure being applied to the ports.
 9. The valveassembly of claim 1 wherein the passage means includes a common inletand exhaust orifice in each end of the chamber, and wherein the valvemeans comprises a solenoid pilot valve operator on each end of the bodywith each valve operator having an independently and selectivelyoperable pilot valve member mounted in the passage means for movementbetween an open position, wherein the orifice at its respective end ofthe chamber is in communication with the supply source and shut off toexhaust, and a closed position, wherein the orifice at its respectiveend of the chamber is connected to exhaust and closed to the supplysource.
 10. The valve assembly of claim 9 wherein the pilot valve memberat each end of the body is normally in closed position with the commoninlet and exhaust orifices each being connected to exhaust uponde-energization of both solenoid pilot valve operators such that thespool may be centered by the spool centering pistons under the returnfluid biases applied thereto.
 11. A valve assembly comprising a bodyhaving a cylindrical chamber with ports including a supply port openinginto the chamber, a valve spool axially reciprocable in the chamberbetween first and second stroke limit positions for controlling fluidflow between ports, the valve spool having a center rest positionintermediate said first and second stroke limit positions, fluid passagemeans in he body for connecting opposite ends of the chamber to exhaustand to the supply port, valve means in the fluid passage means forselectively and independently connecting the ends of the chamber toexhaust and to the supply port, a pair of auxiliary compartments inoperatively aligned adjacent relation to opposite ends of the chamber,and a pair of spool balancing members each having a piston of reducedsize relative to the axial ends of the spool with the pistonsreciprocably mounted within the compartments, respectively, the spoolbalancing members each having an extension projecting into the adjacentend of the chamber for engaging the spool, the fluid passage meansestablishing continuous communication between the supply port and theauxiliary compartments for continuously urging the balancing memberstoward engagement with opposite ends of the spool and applying balancedreturn forces thereon biasing the spool into its center rest position.